The present invention relates to a needle-less injector, preferably a multi-dose injector, wherein a dose of liquid medicament is discharged in a thin jet at sufficient velocity to penetrate the epidermis of the human, animal or plant to be treated, thus to introduce the medicament into the tissues of the subject.
Needle-less injectors are used as an alternative to hypodermic needle type injectors for delivering drugs, vaccines, local anaesthetics and other fluids into the tissues. The medicament is discharged in a jet at high velocity to first puncture the epidermis and thereafter be deposited in the tissues of the subject. A variation is to press the discharge nozzle onto the epidermis and force the liquid at very high pressure through the epidermis.
Prior art devices typically employ a spring-loaded piston pump to generate the injection pressure, in which the piston is retracted against a spring to withdraw fluid from a reservoir. At the end of the piston stroke (which may be adjustable) the piston is disengaged from the retracting mechanism and is urged suddenly by the spring to pressurise and discharge the fluid from the delivery nozzle. The retracting or loading operation may be manual or motorised. In other devices, the piston is driven on the discharge stroke by gas or electric motor instead of a spring.
Manually operated injectors generate a pressure in the medicament of about 100 bars. In operation, the discharge orifice is placed a small distance (about 1 cm) from the epidermis, and the high velocity jet strikes and then penetrates the epidermis (the free jet mode). The principle appears to be that the jet sacrifices some of its kinetic energy to puncture the epidermis, because if the discharge orifice is placed firmly on the skin, and the injector is operated, the liquid is pressurised but has no kinetic energy, ant is unable to pierce the skin. In the free jet mode, medicament is wasted, since some of the liquid is deflected sideways before the puncture is completed, whilst in the contact mode, the epidermis deforms under the pressure of the liquid, which allows all of the liquid to escape without achieving penetration.
Powered injectors generate higher pressures--typically 400 bars or more, which is sufficient to penetrate the epidermis even when the discharge orifice is placed firmly on the skin (the contact mode). However, even in the contact mode a variable quantity of liquid is lost on each injection because the epidermis initially deforms before puncturing, and allows some liquid to escape.
Laboratory tests on both manual and powered injectors often give encouraging results, but in practical situations, such as the vaccination of animals, very variable amounts are injected--frequently over 50% of the vaccine may be wasted--because of hairs and dirt on the injection site, and movement of the animal. The difficulty in achieving successful injections is exacerbated if the subject does not co-operate, as in the case of animals. Premature operation of the injector is common, as is relative movement between the epidermis and orifice which can cause tearing of the epidermis during injection.
Various methods have been proposed to overcome these problems, although in the case of the free jet types, little can be achieved. Powered injectors often employ a vacuum device to suck the epidermis firmly onto the discharge orifice (see WO 82/02835--Cohen, and EP-A-347190 --Finger) and thereby improve the seal between the orifice and skin, and prevent relative movement. Alternatively, a pressure sensitive sleeve on the injector (see U.S. Pat. No. 3,859,996--Mizzy) is placed on the subject, whereby operation of the injector is prevented until the correct contact pressure between the orifice and skin is achieved. The basic aim of such devices is to stretch the epidermis over the discharge orifice, and apply the pressurised medicament at a rate which is higher than the epidermis will deform away from the orifice--i.e. the rate of application of the liquid must be higher than the resonant frequency of the epidermal layer. This condition is not often achieved, and some leakage still occurs.
Powered injectors naturally have available a variety of sensing and control devices to enhance their performance, which are denied to manually operated injectors. However, they are invariably more complex and not easily adapted for portable use. The fact that they generate higher pressures than the manual devices means that their power consumption is high; gas powered injectors require a heavy cylinder of compressed gas, and electrical injectors are often mains powered. Furthermore, the sensing methods used to enable optimum operation are invariably indirect or secondary. For example, U.S. Pat. No. 3,859,996--(Mizzy) discloses a controlled leak method to ensure that the injector orifice is correctly placed at the required pressure on the subject's skin. When the placement conditions are met, the controlled leak is sealed off by contact with the subject's skin, and the pressure within the injector control circuitry rises until a pressure sensitive pilot valve opens to admit high pressure gas to the drive piston. However, the actual pressure of the discharge orifice on the skin is not being measured; a hair or dirt on the sealing face of the controlled leak orifice will prevent or retard the pressure rise in the control circuit, and the operator will unconsciously press the injector harder onto the skin. Also, the timing characteristics may vary because of ineffective sealing, hysteresis of the pressure switch and variations in the gas supply pressure. In other words, the parameters being measured are the effectiveness of the seal of the controlled leak sensor on the skin and the pilot valve response, not the actual pressure of the orifice on the epidermis. Still other devices use a sliding sleeve in contact with the subject, whereby the displacement of the sleeve is used to initiate the injection, but this method measures the load on the sleeve, not on the orifice as required. Again, there can be considerable difficulty when using such injectors on livestock.
It may be seen therefore that whilst needle-less injection potentially is more efficient than hypodermic needle injectors for certain applications, the technique is very dependent on the ability of the operator and the compliance of the subject to be injected. Those injectors that have features designed to reduce the problems tend to be more complex and costly, and less portable.